Process-to-Process Delivery Concepts- UDP, TCP, SCTP
The Internet model has three protocols at the transport layer: UDP, TCP, and SCTP.
The data link layer is responsible for delivery of frames between two neighboring nodes over a link. This is called node-to-node delivery. The network layer is responsible for delivery of datagrams between two hosts. This is called host-to-host delivery. Communication on the Internet is not defined as the exchange of data between two nodes or between two hosts. Real communication takes place between two processes. So that we need process-to-process delivery.
However, at any moment, several processes may be running on the source host and several on the destination host. To complete the delivery, we need a mechanism to deliver data from one of these processes running on the source host to the corresponding process running on the destination host.
The transport layer is responsible for process-to-process delivery-the delivery of a packet, part of a message, from one process to another. The following figure shows these three types of deliveries and their domains.
Client/Server Paradigm:
There are several ways to achieve process-to-process communication; the most common one is through the client/server paradigm. A process on the local host, called a client, needs services from a process usually on the remote host, called a server.
User Datagram Protocol (UDP)
TCP services
TCP Segment
Both processes (client and server) have the same name. For example, to get the day and time from a remote machine, we need a Daytime client process running on the local host and a Daytime server process running on a remote machine.
A remote computer can run several server programs at the same time, just as local computers can run one or more client programs at the same time. For communication, we must define the following:
1. Local host
2. Local process
3. Remote host
4. Remote process
Addressing
Whenever we need to deliver something to one specific destination among many, we need an address.
At the data link layer, we need a MAC address to choose one node among several nodes if the connection is not point-to-point. A frame in the data link layer needs a destination MAC address for delivery and a source address for the next node's reply.
At the network layer, we need an IP address to choose one host among millions. A datagram in the network layer needs a destination IP address for delivery and a source IP address for the destination's reply.
At the transport layer, we need a transport layer address, called a port number, to choose among multiple processes running on the destination host. The destination port number is needed for delivery; the source port number is needed for the reply.
In the Internet model, the port numbers are 16-bit integers between 0 and 65,535. The client program defines itself with a port number, chosen randomly by the transport layer software running on the client host. This is the ephemeral port number.
The server process must also define itself with a port number. This port number, however, cannot be chosen randomly. If the computer at the server site runs a server process and assigns a random number as the port number, the process at the client site that wants to access that server and use its services will not know the port number.
Every client process knows the well-known port number of the corresponding server process. For example, while the Daytime client process, discussed above, can use an ephemeral (temporary) port number 52,000 to identify itself, the Daytime server process must use the well-known (permanent) port number 13.
It should be clear by now that the IP addresses and port numbers play different roles in selecting the final destination of data. The destination IP address defines the host among the different hosts in the world. After the host has been selected, the port number defines one of the processes on this particular host.
IANA Ranges:
The lANA (Internet Assigned Number Authority) has divided the port numbers into three ranges: well known, registered, and dynamic (or private).
• Well-known ports: The ports ranging from 0 to 1023 are assigned and controlled by lANA. These are the well-known ports.
• Registered ports: The ports ranging from 1024 to 49,151 are not assigned or controlled by lANA. They can only be registered with lANA to prevent duplication.
• Dynamic ports: The ports ranging from 49,152 to 65,535 are neither controlled nor registered. They can be used by any process. These are the ephemeral ports.
Socket Addresses:
Process-to-process delivery needs two identifiers, IP address and the port number, at each end to make a connection. The combination of an IP address and a port number is called a socket address. The client socket address defines the client process uniquely just as the server socket address defines the server process uniquely which is represented in the following figure.
Multiplexing and Demultiplexing:
The addressing mechanism allows multiplexing and demultiplexing by the transport layer.
Multiplexing:
At the sender site, there may be several processes that need to send packets. However, there is only one transport layer protocol at any time. This is a many-to-one relationship and requires multiplexing. The protocol accepts messages from different processes, differentiated by their assigned port numbers. After adding the header, the transport layer passes the packet to the network layer.
Demultiplexing:
At the receiver site, the relationship is one-to-many and requires demultiplexing. The transport layer receives datagrams from the network layer. After error checking and dropping of the header, the transport layer delivers each message to the appropriate process based on the port number.
Connectionless Versus Connection-Oriented Service
A transport layer protocol can either be connectionless or connection-oriented.
Connectionless Service:
In a connectionless service, the packets are sent from one party to another with no need for connection establishment or connection release. The packets are not numbered; they may be delayed or lost or may arrive out of sequence. There is no acknowledgment either. UDP, is connectionless.
Connection-Oriented Service:
In a connection-oriented service, a connection is first established between the sender and the receiver. Data are transferred. At the end, the connection is released. The TCP and SCTP are connection-oriented protocols.
Reliable Versus Unreliable:
The transport layer service can be reliable or unreliable. If the application layer program needs reliability, we use a reliable transport layer protocol by implementing flow and error control at the transport layer.
If the application program does not need reliability because it uses its own flow and error control mechanism or it needs fast service or the nature of the service does not demand flow and error control (real-time applications), then an unreliable protocol can be used. In the Internet, UDP is connectionless and unreliable; TCP and SCTP are connection oriented and reliable.
For Further Reading:
How To Create a TCP Connection?
Flow Control in TCP
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